1. Field of the Invention
The present invention relates to liquid crystal light valve color projection systems, and more particularly to an apparatus and method for separating and combining colors in a single projection lens liquid crystal light valve projection system.
2. Discussion
Optical projection systems, such as video projector systems are used for projecting images onto a screen. Since projection systems can project images at a wide range of sizes (within certain limitations), these systems can potentially yield larger images than conventional imaging systems such as CRTs. In the most common approach, video projection systems employ three CRTs, each projecting one of the primary colors (red, green or blue) onto a screen. However, as the size of the projected image is increased, its brightness is reduced. To overcome this and other problems, projection systems incorporating liquid crystal light valves (LCLVs) have allowed important advances by providing increased light output. LCLVs have been used in many applications, such as those where a very large projection screen must be illuminated by a projector occupying a very small volume, and also in very high brightness projection display systems. In general, where the intensity of light emitted by a conventional screen such as a cathode ray tube is not adequate because of high ambient light conditions, or where very large projection screens are employed in limited space, liquid crystal color display projection systems are preferred.
Liquid crystal light valve projection systems are generally either of the transmission type (active-matrix) or the reflective type. In transmission (active matrix) liquid crystal light valves, each liquid crystal light valve individually modulates its respective monochromatic beam over a spatial array of pixels, and the beams are then combined with a multiplexer or combining prism and projected as a single combined beam of appropriate color. The combined beam bears information imposed thereon by computer controlled modulation of the several liquid crystal light valves. The combined beam is fed through a projection lens onto the front or back surface of a diffuse display screen to provide appropriate display on the screen surface.
In some active-matrix liquid crystal projectors, dichroic mirrors separate white light emitted by a high intensity halogen lamp into three monochromatic beams, red, blue and green. These are passed through the individual liquid crystal modulator panels, and the resulting modulated monochromatic light beams are combined into a single multi-color beam by an X-prism, and then projected through a set of lenses onto the screen. Such systems necessarily employ large numbers of optical relay systems or optical elements, such as lenses and mirrors, to split and distribute reading light from the source lamp to the various liquid crystal modules. Further, these relay systems require significant amounts of space to position the several dichroic reflecting mirrors, which in and of themselves require optical relay systems to provide longer focal lengths that enable proper positioning of such additional components. Physical size of the system is therefore greatly increased.
Similar problems, e.g., large numbers of components and increased space requirements, exist in those projection systems employing reflective liquid crystal light valves. In some reflective liquid crystal light valve systems reading light from a high powered source is polarized by a polarizing beam splitting prism designed to reflect "S" polarized light (in which the polarization axis, namely the E field vector, is parallel to the plane of incidence) and to transmit "P" polarized light (in which the polarization axis, the E field vector is perpendicular to the plane of incidence). The "S" polarization component of the reading light is reflected to the light valve, which, when activated by an image from a writing light source, such as a cathode ray tube, reflects the polarized light and rotates its polarization 90.degree. so that it becomes "P" polarized light. The "P" polarization component is transmitted through the prism to the projection lens. When such reflective liquid crystal light valves are employed in a color projection system, the input light path must be lengthened by additional relay optics to provide for separation of the reading light source into three color components and prepolarization of the several color components.
Often LCLV projection systems are configured with three separate projection lenses, one for each of the red, green and blue primary colors. However, there are a number of disadvantages with the use of three projection lenses. Three projection lenses add significantly to the number of optical components as well as to the overall size and cost of the system. Further, the image produced by the three lens projector must be converged repeatedly if the distance between the projector and the screen is subsequently varied. That is, the three individual projector lenses are angled toward the screen and will achieve exact convergence at only one projector lens to screen distance. However, if the screen is further away the projector lenses will have to be angled at less of an angle and if the screen is moved closer the lenses will have to be turned at greater angles. Alternatively, it is possible to move the image on the CRT instead of changing the angle of the lens itself. Either way, however, any change in the distance between the screen and the projector requires considerable adjustment. Similarly, trapezoidal correction differs for each projection lens and must be individually corrected as well.
Single projection lens systems offer the potential for a reduced size and lower cost because they can be made using fewer optical components. They also eliminate the above-discussed need to repetitively converge the three primary color images. However, the advantages of existing single projection lens systems are counterbalanced by at least two primary disadvantages. First, single lens video projector systems are characterized by large light losses because of the use of series trim filters and rejection of one light polarization for each color.
That is, since it is desirable to have spectral notches between the blue, green and red primary colors, trim filters are necessary to produce the desired spectral separation between red, green and blue to achieve a desired level of color saturation. But these involve not insignificant light losses. Also, the rejection of one light polarization for each color is a problem because such systems generally use one linear polarization of light for one primary color and the orthogonal polarization for the other two primary colors. In these systems, unpolarized light is the required input to the polarizer/analyzer. Since one of the two orthogonal polarizations of light is rejected for each primary color, the system is, at best, only 50% efficient in terms of light use.
A second disadvantage with existing single lens systems is the necessity to orient the liquid crystal light valves at angles with respect to one another. This requirement means that the three CRTs that write on the liquid crystal light valves cannot be arranged in a very compact manner. That is, canting the elongated CRTs at angles takes up far more space than would a roughly parallel CRT arrangement. While it might be possible to decrease the space requirements by incorporating a parallel CRT arrangement, this would require a lengthened air optical path utilizing folding mirrors which would necessitate a projection lens with a much longer back focal length. This is not economically feasible in most systems since the diameter and cost of a lens increases significantly as the back focal length increases. Thus, this cost constraint significantly limits the ability to design a more compact projector optical configuration.
An additional problem with liquid crystal light valve projection systems which utilize dichroic filter configurations is the resulting astigmatism in the projected image that results whenever an angled dichroic filter configuration is used in air.
Other problems arise with conventional liquid crystal light valve systems in general. For example, the complex optical components are generally exposed to air. This allows dust and other airborne particles in to the critical light valve optical imaging path. This results in particle and haze contamination on the optical surfaces in that path. Particle and haze contamination will detract from the contrast ratio and the efficiency of light use.
Thus, it would be desirable to provide a liquid crystal light valve projection system which has a simplified construction to minimize the number of optical components as well as the size and cost of the overall system. Further, it would be desirable to provide such a liquid crystal light valve system which makes relatively efficient use of input light to provide a high output light projection system. It would also be desirable to provide a system with the above features that also provides adequate spectral notches between the red, green and blue spectra to provide a high degree of color saturation with a low amount of distortion such as astigmatism. It would further be desirable to provide the above features in a system that minimizes the particle and haze contamination of its optical components.